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https://hdl.handle.net/2142/70178
Description
Title
Homogeneous Cobalt Catalyzed Oxidations
Author(s)
Zombeck, Alan
Issue Date
1981
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Inorganic
Abstract
Mechanistic studies of the oxidation of various substituted phenols to benzoquinones by Co(II) bis(salicylidene-(gamma)-iminopropyl)-methylamine, CoSalMDPT are described in Chapter I. The reaction is first order in {O(,2)}, {substrate}, and {Co}. A series of experiments are reported to provide strong support for a mechanistic scheme that involves reaction of coordinated dioxygen in two distinct steps of the mechanism. Coordination of O(,2) to this cobalt(II) complex enhances the ability of the dioxygen to abstract hydrogen atoms from the phenol to produce phenoxy radicals. This proposed hydrogen atom abstraction step is supported by hydrogen bonding and EPR studies. A deuterium isotope study showed this step is not rate determining. Further reaction of the phenoxy radical with a Co-O(,2) species is supported by several pieces of evidence. If the phenoxy radicals are generated simultaneously via some other reaction in the presence of a cobalt complex that does not bind O(,2), no benzoquinone is formed. Also, the EPR spectrum of a cobalt-peroxy-benzoquinone complex which was isolated from the reaction mixture, displayed an overlapping Co-O(,2) and phenoxy radical signal. This suggests an equilibrium between a Co-O(,2) species and a phenoxy radical. Catalyst deactivation studies revealed benzoquinone and water do not inhibit the reaction, but rather acids produced as oxidized side products eventually deactivate the catalyst. Thus, the addition of non-coordinating bases to the reaction medium greatly enhanced the number of turnovers.
The CoSalMDPT complex catalyzes the oxidation of terminal olefins to the corresponding methyl ketones and secondary alcohols. Chapter II describes the investigation of this reaction which indicates a unique mechanism other than autoxidation. The oxidation of terminal olefins by CoSalMDPT is very solvent dependent, only ethanol is an acceptable solvent. The reaction is independent of O(,2) pressure and is first order in {Co} and {1-hexene}. No induction period is observed and the reaction is not inhibited by free radical traps. Styrene is easily oxidized by CoSalMDPT whereas cyclohexene is not. Typical autoxidation catalysts do not catalyze this reaction under identical conditions used with CoSalMDPT. A synergistic enhanced reaction rate is observed with the addition of rhodium to the CoSalMDPT catalyzed oxidation. The cobalt catalyst does not oxidize 2-hexanol, thus suggesting a mechanism were both oxidation products are derived from a common intermediate.
Chapter III describes the preparation and characterization of several cobalt complexes supported on silica gel. An EPR study of these supported complexes demonstrated the site isolation properties of silica gel. Several unsuccessful attempts are discussed to prepare silica gel supported CoSalMDPT. Finally, the preparation of a silica gel ion exchange resin is described.
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